The need for covert communications, military and civilian, is both historic and ever-present. A communication may generally be considered covert if an intruder is unaware of its presence. In measuring covertness, one may consider many parameters such as: (1) probability of interception (intruder's ability to receive a portion of a communication of which he is aware); (2) jamming (intruder's ability to interfere or limit transmission of information); and (3) spoofing (intruder's ability to interject false information without it being recognized as false).
In attempting to avoid detection by an intruder, a system should attempt to minimize the above-named measures. Any communication system operates within a 5-dimensional space. Thus, a covert system may utilize up to five degrees of freedom to "hide" signal. The five dimensions available are time, frequency, and the three spatial dimensions. The veiling of the signal is commonly accomplished by (1) concentrating all signal energy into a small portion of the total volume of space in the hope that an intruder will not stumble across it or (2) moving the signal energy in some predetermined manner at a rapid rate through as much of the volume (of all five dimensions) as possible. In the latter instance, the intruder is required to observe the total spatial volume (rather than a small portion), thus incurring a reduction in receiver sensitivity. Though effective in many applications, such systems exploit only two of the above-named communication dimensions, frequency and time. In addition, the latter dimension is exploited in only one direction. The three spatial degrees of freedom remain unutilized for concealment due to the finite size of the receiver's antenna. Thus, these techniques are effectively limited to about one and one half dimensions of the five potentially available for hiding a communication.
Optical communications, on the other hand, maintain covertness through the concentration of signal energy into a small portion of space. Though not restricted solely to communication systems, it is well known that a laser system may conveniently achieve a very small energy volume relative to a comparable RF system. As optical systems also possess the ability to take advantage of the non-spatial communication dimensions, laser communication systems exercise all the degrees of freedom available to RF communication systems and more. As an example of the spatial covertness which may be achieved by a laser system, the (three dimensional) volume into which energy is directed by a 10 centimeter antenna at 37.5 GHz is 64,000 times as large as the energy volume produced by the same size antenna (10 centimeter diameter telescope) operating at 30,000 GHz (10 micrometer wavelength). An interceptor utilizing omnidirectional receiving antennas would be assured of location within the radiation field of such a transmitter, allowing the conduct of a systematic frequency search with reasonable assurance of detection. Contrariwise, due to the nature of laser communication, not only does the potential interceptor have to locate himself properly in frequency, time and space, he must also properly orient his antenna field-of-view to receive the transmitted energy. Thus, his search actually has seven degrees of freedom: the five previously discussed, plus the azimuth and elevation directions of his receiving antenna's field-of-view. This presents a huge, multifaceted problem. (An interceptor utilizing an optical heterodyne receiver has a field-of-view limited to approximately 2.4.lambda..div. where .lambda. is the wavelength and d is the diameter of the antenna; thus, a 10 centimeter receiving antenna is limited to a field-of-view on the order of 0.014 degrees).
The very desirable directionality of the optical (laser) communication mode, which facilitates the concealment of optical transmissions in space, gives rise to the predictable difficulties of operation found to exist when the field-of-view communications are transmitted between transceivers (transmitter/receivers) having unstable platforms. This situation has recently arisen with regard to the development of lightweight, hand-held optical communicator systems. The possibility of developing a practical line-of-sight optical communicator operable over a range of 5 to 10 miles arose with the development of the gallium arsenide (GaAs) injection laser diode. [A hand-held optical communicator presently available is described in "Gallium Arsenide Laser Communicators for Hand-Held Voice or Fixed-Base Voice/Data Optical Communications" by Robert J. Cinzori (Procs. Sixth Conference on Laser Technology, Department of Defense (1974))]. Optimum covertness is achieved with such a system when the solid angle of the transmission in space is a minimum. The maintenance of a small angle of transmission leaves little margin for the errors incurred through receiver platform instability. The interrelationship of range and energy density imposes a physical limit (in addition to the security considerations which dictate minimal spatial distribution of a signal) upon the solid angle of the laser transmission. (It has been found that a communication range of seven miles may be achieved by a present day hand-held optical communicator having a transmission beamwidth of one and one-half degrees. This range is degraded to about one mile when the aperture is increased to four degrees).
The maintenance of transmission between two optical communicators of the type described above is disclosed in pending U.S. patent Ser. No. 95,178 filed on Nov. 16, 1979 by Richard A. Dye and titled "Self-Aligning Laser Communicator Utilizing Reciprocal Tracking". This application, which is the property of the assignee herein, discloses an automatic technique and apparatus therefor based upon the art of reciprocal tracking. The art disclosed in the pending patent application essentially utilizes a quadrature (four-piece) detector and associated circuitry which processes the information inherent in an unequal distribution of received energy upon such detector to continually reposition the reciprocal-tracking transceivers of the communicators by electro-mechanical means. This tracking technique and apparatus is effective provided a rough line-of-sight is maintained between the communicators so that incoming transmissions continue to fall with some distribution upon the quadrature detectors of the communicators. Unfortunately, the maintenance of a rough line-of-sight may be difficult in various important applications such as shipboard platforms, battlefields, etc., and nearly impossible to establish at night and in other low-visibility situations.
Once a "line-of-sight" is established at night, it may be subject to periodic "drop-out"--a result of the difficulty, if not impossibility, of attempting to hand-hold a communicator to within .+-.1.degree. accuracy. Thus, the "shaky" base provided by the human operator will often drive the device to the edge of communication--a suboptimal field-of-view alignment--whereby the automatic tracking apparatus of the Dye patent application must continually reposition the transceiver. This apparatus becomes useless once the line-of-sight is lost--a distinct possibility if such line-of-sight is on the edge of the communication.